In the operation, maintenance and repair of an internal combustion engine, it is often difficult to rapidly determine the specific cause of improper engine operation. This is particularly true in the case of the electrical ignition system, where faults in the high voltage section are often difficult to diagnose. Typical faults in such systems include fouled or shorted spark plugs as well as open or arcing cables. There are commercially available instruments to aid the mechanic in the diagnosis of ignition faults, the most successful to date being the modern engine analyzer which displays the high voltage waveforms for all of the spark plugs on a cathode ray tube. A highly trained mechanic can observe the variations in shape of the individual waveforms to determine which spark plug or cable is faulty.
It would be desirable to have instruments which are as effective as the cathode ray tube analyzer in diagnosing ignition faults, but which required much less training and expertise to operate and interpret. The present invention is useful in that regard; i.e. information is processed as it is detected so that it may be presented to the mechanic in a simplified form which requires minimal skill for correct interpretation.
Perhaps the most important aspect of the high voltage spark plug waveforms is a very rapid positive excursion after the voltage has reached levels in the range from about -7000 to -12,000 volts. This rapid (positive) rate of change towards zero voltage is a direct result of an arc which discharges the cable. In the case of a fouled spark plug, open cable or generally inoperative high voltage source, the arc will not occur and the rapid change in voltage will be absent.
In mathematical terminology, the rate of change of voltage is a derivative of voltage with respect to time, symbolized as dV/dt, where V represents voltage and t is time. In a typical case, the spark plug arc may cause the voltage to change from -10,000 volts to zero volts within approximately 0.2 microseconds. This rather rapid change in spark plug voltage corresponds to a derivative of 5.times.10.sup.10 volts per second.
Since this high dV/dt is characteristic of an arc discharge, most of the common ignition failures are characterized by dV/dt which is much lower. See FIG. 3 for a comparison of waveforms; note the greatly reduced magnitude of the derivative when an arc does not occur.
The present invention obtains this derivative in an efficient manner, and provides means for processing the information about the magnitude of dV/dt so that it may be presented in a simple pass-fail format.
The most essential features of the present invention include:
(a) An electric probe which is capacitively coupled to the spark plug or spark plug cable. PA1 (b) Circuitry to combine with the capacitance developed between the probe and the ignition cable or spark plug (such capacitance hereafter will be referred to as "probe capacitance") to form a voltage differentiating circuit. PA1 (c) Means to provide electric shielding for the probe. PA1 (d) Means to detect the level of the differentiated ignition voltage and further means to provide the operator with a visual indication of the amplitude of the differentiated voltage.
For example, the shielded probe may consist of two electrically conductive half-cylinders which are arranged to have a common axis. The inner cylindrical half-section would serve as the electric probe mentioned in (a) above while the outer half-cylinder would provide the electric shielding mentioned in (c) above. Probe capacitance, as stated in (b) above, would be that capacitance between the inner half-cylinder and the ignition component where voltage was being sensed. If the spark plug cable were being sensed by the probe, probe capacitance would essentially be between the spark plug cable conductor and the inner half-cylinder. Recall that probe capacitance is used not only to sense voltage, but also as a key element in a differentiating circuit. The differentiating circuit may combine the probe capacitance with combinations of resistance, inductance or operational amplifiers or the like to achieve the differentiating function.
Since probe capacitance is crucial to the differentiating function of the present invention, it is worthwhile to consider its geometry and typical values. The probe sensing electrode, which is the inner half-cylinder in the previous discussion, is placed around a spark plug cable so that its axis is coincident with the cable conductor. The probe capacitance for this arrangement is approximately: EQU C=[.pi.Lel n(r.sub.2 /r.sub.1)] Farads
where L is the length of the half-cylinder electrode in meters, e is the dielectric constant of the medium between the electrode and the cable conductor and r.sub.1 and r.sub.2 are the radius of the cable conductor and the probe electrode respectively. In a typical case, where L is 0.05 meters, e is 3.times.8.8.times.10.sup.-12 Farads per meter, r.sub.1 is 0.5 mm and r.sub.2 is 4 mm, the probe capacitance is 2 pF.
If this 2 pF capacitance is connected in series with 1000 Ohms resistance to form a differentiator, and if dV/dt is 5.times.10.sup.10 volts per second, the peak differentiated voltage developed across the resistor would be approximately EQU RC(dV/dt)=100 volts
In this simple calculation, where R is the 1000 Ohm resistor and C is the 2 pF probe capacitance, one obtains the "order of magnitude" response for this type of differentiator. Actual response measured in a test settup with similar components ranged from 40 to 60 volts peak. When a simple differentiator is used, it is necessary to make the RC time constant smaller than the period of the highest frequency (in radians per second) that is to be differentiated. If an operational amplifier amplifier is used, as shown in FIG. 5, primary limitations on differentiation will be due to the upper break points or "poles" associated with the operational amplifier open loop gain or with the stabilizing circuitry. Complete circuitry utilizing an RC differentiator is shown in FIG. 1 and FIG. 4. FIG. 4 has the added feature of self-checking circuitry which operates when the power switch is depressed. This feature is realized by connection of one side of the pulse-stretching capacitor to the output of the power switch. This leads to a positive transient on the gate of the input transistor whenever the power switch is depressed, resulting in an output from the light emitting diode if the circuitry is operative and if the battery is sufficiently charged.
An operational amplifier is shown in FIG. 5; note the presence of resistance and capacitance elements to stabilize the differentiator.
Operation of the differentiating circuit can be enhanced by causing it to be especially sensitive to a more narrow band of frequencies. This can be accomplished by the addition of one or more inductive elements to resonate with probe or cable capacitance; one example is shown in FIG. 6. This inductive element (in FIG. 6), in combination with the probe capacitance can form a series resonant circuit whose bandwidth may be adjusted by setting the value of resistance in series with the inductor. This arrangement will enhance circuit selectivity and thereby reduce the sensitivity of the circuit to undesirable noise signals. By example, the frequencies present on the spark plug cable as a result of arc discharge might fall within the range of about 1 to 10 MHz, so that it would be useful to design for a center frequency of 3 MHz. For a series (probe) capacitance of 2 pF, the value of inductance required would be: EQU L=(4.pi..sup.2 f.sup.2 C).sup.-1 =1.41 mHy
Alternately, it is also possible to use the inductor to form a parallel resonant circuit in combination with cable capacitance. A typical value for cable capacitance is about 20 pF; the inductance required for a 3 MHz parallel resonance would be 0.141 mHy.
The choice of whether to enhance selectivity with either series or parallel resonance (or a combination of both) depends upon the relative values of probe capacitance and cable capacitance. If the two capacitance values could be adjusted, one could realize both parallel and series resonance at the same or at relatively close frequencies.
The invention described herein has been constructed and tested in several versions, both on a mock-up of a standard automotive "coil and breaker point" ignition system as well as on a variety of ignitions systems in automobiles. The most successful probe was constructed of two half-cylinders of metal which were separated by epoxy insulation and connected to the electronic monitoring circuitry by a short section of 50-Ohm cable. The chassis housing the electronic circuitry was metal, since earlier experiments with a plastic chassis did not provide enough shielding for the input section of the sensing/differentiating circuitry.
Tests conducted on the simulated ignition circuitry and on various automobiles proved that the present invention is quite capable of detecting spark plugs with fouled, shorted or eroded gaps. The invention is also capable of detecting any condition which isolates high voltage from the spark plug or the section of cable under test; this may include an open cable or a faulty autotransformer or a failed component at the lower voltage levels of the ignition system.
Since the present invention is effectively a differentiating arc-detector, it is less efficient at detection of spark plug cables which are arcing to ground than in detecting other more common faults. Some capability for detecting arcing cables (by not responding to the resultant dV/dt), may be achieved by adjustment of detector sensitivity or by displaying a continuous scale of dV/dt for observation by the mechanic. This is due to the fact that arcs which are remote from the spark plug (where the detector probe will normally be located), must discharge the probe capacitance through the spark plug cable impedance and therefore result in a lower dV/dt than when the arc is near the region of measurement.
The present invention may be used by the mechanic in a variety of ways, depending upon the exact configuration of the instrument. When a single differentiator and display unit is housed in a small chassis, the mechanic may check each spark plug or spark plug cable in a serial manner to determine which are likely to be faulty. If a number of the differentiator probes are available (eight for example), the mechanic may view the results on all spark plugs simultaneously. In this case, one might have a number of monitors and display modules which was equal to the number of shielded differentiator probes. Alternately, one might wish to multiplex the data for display or for input into a microprocessor-based data analysis system.